Possibility to use Postgres as a backend.

It would be nice to support Postgres as one of the backends for attachments using "large objects".
More info about them can be found here: http://www.postgresql.org/docs/9.3/static/largeobjects.html

Not sure if this should be included in this project or should go as a separate gem. 

Please review this.
Converting to integer would be enough in this case. but using http://www.rubydoc.info/gems/pg/PG/Connection#exec_params-instance_method is probably better for performance